Doing concepts requires a very fast turn around, and typically you do not face technical limitations because the idea is the only thing that matters. This means you can use a full pallette of tools and techniques and need deliver a series of renders. Moi is perfact for that!

This is only the first phase in a production pipeline though. Concept Modeling has considerably freedom because its not expected to be used in the pipeline. The idea will usually be sent to a small army of modelers and texture artists that will do the final pass on it. Its a creative industry, so the initial idea will see many iterations thoughout the entire production so everything will need to be flexible to the very end. Very few people actually use Nurbs modelling.

Here are things to watch for:

File Size:
Nurbs modelling is final resolution and cannot be subdivided, which makes particularly very dense geometry once it is converted to polygons. Not an issue really, but it can create a bottle neck or even break it if assets are getting to be +1 GB in file sizes. VFX studios prefer and often expect very clean and light weight models.

Topology:
Doing a sculpting pass using Zbrush/Mudbox will require a tedious clean up. Poles can present a serious problem, as some applications will not accept the mesh entirely. Mari and Mudbox will report these as errors. Using Displacement will also be problem if the geometry does not have a clean topology.

Surface Normals:

Are locked on the mesh, which is fine as long you dont do any modifications to the geometry once it makes way into other applications as a polygonal mesh. This requires a strict SOLID surfaces only workflow and any post-mirrored goemetry will require fixing the normals.

UVs:
Nurbs models do create a basic UVs and this is quite useful, but will require cleanup. Using Default, creates UV seams at all of the patch edges, which can be a problem. I take advantage of this within Maya though, by selecting the UV shells and reprojecting them. Chamfered edges will require manual cleanup.